Motor Grounding NEC 250.122(D)(2)

[neceltic]

I am a little confused by motor circuit grounding as listed in 250.122(D). 250.122(D)(1) is straightforward and points to using Table 250.122 for motor ground sizing. However, 250.122(D)(2) seems to say that if one has an instantaneous trip breaker then the ground has to be sized for the maximum fuse allowed using 430.52(C)(1). This would increase the grounds in larger HP motors. The particular problem is my customer uses cabled products so an upsized ground would mean a non-standard cabled product. The AHJ's opinion is that (D)(2) takes precedence over (D)(1). I haven't heard of this from any other inspectors, customers or vendors. Has anyone ran across this before? Explanation of the code's intent?

 

[infinity]

Here's 250.122(D)(2) which modifies the requirement of 250.122(D)(1) for instantaneous trip devices.

250.122 Size of Equipment Grounding Conductors.
(D) Motor Circuits. Equipment grounding conductors for motor circuits shall be sized in accordance with (D)(1) or (D)(2).
(2) Instantaneous-Trip Circuit Breaker and Motor Short-Circuit Protector. Where the overcurrent device is an instantaneous-trip circuit breaker or a motor short-circuit protector, the equipment grounding conductor shall be sized not smaller than that given by 250.122(A) using the maximum permitted rating of a dual element time-delay fuse selected for branch-circuit short-circuit and ground-fault protection in accordance with 430.52(C)(1), Exception No. 1.

 

[neceltic]

Motor Grounding

Motor Grounding

Sounds like the AHJs interpretation is correct in that if we have a MCC bucket with a thermal mag breaker, then we have to upsize the ground as if the maximum fuse was installed?

 

[don_resqcapt19]

Sounds like the AHJs interpretation is correct in that if we have a MCC bucket with a thermal mag breaker, then we have to upsize the ground as if the maximum fuse was installed?

In the case of a thermal mag breaker the EGC is sized based on the breaker size. It is only when you use a an instantaneous trip breaker that you have to size the EGC based on the maximum permitted fuse size. Note that this second option will most often result in a smaller EGC than when the breaker is a thermal mag breaker. Also note that 250.122(A) applies and the EGC is not required to be larger than the motor circuit conductors.